Pollution control devices such as catalytic converters for gasoline engines have been known for over 30 years. In the last few years, more stringent regulations for diesel vehicles have resulted in a rapid increase of other pollution control devices including diesel oxidation catalysts (DOC's), diesel particulate filters (DPF's), and selective catalytic reduction devices (SCR's). The pollution control devices typically comprise a metal housing or casing with a pollution control element securely mounted within the casing by a resilient and flexible mounting mat. Catalytic converters, including diesel oxidation converters, contain a catalyst, which is typically coated on a monolithic structure. The monolithic structures are typically ceramic, although metal monoliths are also known. The catalyst in a gasoline engine oxidizes carbon monoxide and hydrocarbons and reduces the oxides of nitrogen to control atmospheric pollution. A diesel oxidation catalyst oxidizes the soluble organic fraction of soot particles as well as any carbon monoxide present.
Diesel particulate filters or traps are typically wall-flow filters, which have honeycombed, monolithic structures that are typically made from porous crystalline ceramic materials. Alternate cells of the honeycombed structure are typically plugged such that exhaust gas enters in one cell and is forced through the porous wall to an adjacent cell where it can exit the structure. In this way, the small soot particles that are present in diesel exhaust are collected. From time to time, the temperature of the exhaust gas is increased above the incineration temperature of the soot particles so that they are burned. This process is called “regeneration.”
Selective catalytic reducers are similar in structure and in function (i.e., reduce NOx) to catalytic converters. A gaseous or liquid reductant (generally ammonia or urea) is added to the exhaust gas before reaching the selective catalytic reducer monolith. The mixed gases cause a reaction between the NOx emissions and the ammonia or urea. The reaction converters the NOx emissions into pure nitrogen and oxygen.
The monoliths, and in particular the ceramic pollution control monoliths, used in pollution control devices are fragile, and susceptible to vibration or shock damage and breakage. They have a coefficient of thermal expansion generally an order of magnitude less than the metal housing that contains them. This means that as the pollution control device is heated the gap between the inside periphery wall of the housing and the outer wall of the monolith increases. Even though the metallic housing undergoes a smaller temperature change due to the insulating effect of the mat, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the ceramic monolith. Such thermal cycling occurs hundreds of times during the life and use of the pollution control device.
To avoid damage to the ceramic monoliths from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and metal housing (thereby bypassing the catalyst), mounting mats are disposed between the ceramic monolith and metal housing. These mats exert sufficient pressure to hold the monolith in place over the desired temperature range but not so much pressure as to damage the ceramic monolith.
Known mats include intumescent sheet materials comprised of ceramic fibers, intumescent materials and organic and/or inorganic binders. In recent years, non-intumescent mats, especially those comprised of polycrystalline ceramic fibers and binder, have been used. Polycrystalline fibers are much more expensive than (melt-formed) amorphous refractory ceramic fibers (i.e., a fiber that is melt formed and has not been post processed by heat treating to either anneal or crystallize the fiber, so as to be substantially crystalline free, meaning that no crystallinity is detected by powder x-ray diffraction) and, therefore, mats using these fibers are used where deemed absolutely necessary such as with ultra thin-wall monoliths or for pollution control devices that are exposed to water during use (due to filter cleaning, water condensation, rain water from vertical stacks, etc.). Water can have a deleterious effect on certain intumescent mounting materials. Non-intumescent mats comprising only amorphous refractory ceramic fibers generally lack the necessary holding force to function as a mounting mat. Performance of amorphous refractory ceramic fibers can be improved, but it typically requires expensive shot removal and heat treatment to high temperature to at least partially crystallize the fibers. Mats comprising magnesium aluminum silicate glass fibers have also been tried, but generally lack sufficient temperature capability.